The speedo is an easy one to troubleshoot. Like Fletch said the guage is fed by a line connected to the pitot. Start at the pitot. Disconnect the hose at the pitot. Inspect the pitot to ensure that it is not clogged or damaged. Then trace the tubing on the inside of the boat. In some cases the tubing may lead to a brass tube. If yours has this then remove the tubing from each end of the tube. Make sure air flows through the end of the tube leading out to the pitot. Make sure air flows through the tube. Remove the tubing at the back of the speedometer. Make sure air travels through the tubing all the way to/from the speedometer. If your plumbing is in tact to the speedo, then it is the speedo (ulikely - usually the problem is between there someplace).

As for the guages jumping at bumps that is probably a grounding issue. Little tougher, lots of wires to trace and check their connections. Clean, repair, replace as required.